The invention relates to a method for the manufacture of a ceramic hollow fiber based on nanoscale oxide particles, preferably yttrium stabilized zirconium oxide, zirconium oxide, titanium dioxide, silica and alumina as well as ceramic hollow fibers manufactured according to this method.
Ceramic fibers are achieving more and more industrial significance wherein especially whole ceramic fibers of alumina are already available in the marketplace. Thus, the firms 3M, Mitsui, Sumitomo and Toyoba already offer continuous aluminum oxide ceramics in price ranges between 400 and 1800 US$/kg. Short ceramic fibers with lengths in the range from 1 μm are of lesser industrial significance due to the fact that these fibers may no longer be processed in Germany for example, based on their lung respirability. New development trends are coming to the fore in the area of the ceramic hollow fibers which are establishing the ceramic hollow fiber principle in all areas in which whole fibers are already established and additionally developing other market segments.
Ceramic hollow fibers, although not yet commercially available, however are the subject of actual developments in many research establishments. In comparison to whole fibers, hollow fibers possess greater bending strength and a higher insulation factor which together with lower material usages, from about 40 to 60 wt %, is accompanied by savings in weight for equal volumes. Added to that is the fact that hollow bodies can be cooled from the inside, and for example heat, and internal materials etc. can be easily transported to the exterior.
The most important fields of application of ceramic hollow fibers are in the areas of metal, polymer and ceramic sheathings, artificial organs, fiber optics, ceramic membranes, solid electrolyte for the (SOFC) fuel cell, tissue engineering, the textile industry, and the manufacture of extremely light, temperature resistive ceramic structural elements such as heat shields or brake systems, which can purposely dissipate heat.
In contrast to planar structures, with the hollow fibers three dimensional and also rotationally symmetrical structures are produced, which also permit flexible use in numerous applications of microsystem technology.
For the economic efficiency of the uses proposed herein it is crucial that the manufactured hollow fibers can be densely packed and as a result guarantee a high surface to volume ratio, ideally the hollow fibers are very small and plastically formable. In the area of fuel cells for example, very large external diameter (in the range of several mm) Y—ZrO2 hollow fibers usable as electrolyte lead to high cathode resistances and to small power densities. In the area of filter membranes, the per unit volume available for stationary specific surface area must be very high, in order to still be able to realize efficient filter systems. Attempts have not succeeded in producing hollow fibers by spinning solutions, gels or brines of the corresponding starting materials, which are then transformed by diverse chemical reactions and physical processes to ceramic fibers. These attempts were limited in that the necessary starting substances are not always available, or that not every wanted phase can be prepared by pyrolytic decomposition and phase conversion but, that by sintering of the hollow fibers (if one wants to prepare a ceramic hollow fiber one must remove the organic processing agent) the shrinkage and thereby the stresses in the hollow fiber becomes so great that the hollow fiber shatters. Up to now, it appears as if by such means only the preparation of SiO2 as hollow glass fibers has succeeded in large quantities (Fraunhofer Institute for Silicate Research (Fraunhofer Institut für Silicatforschung) in Würzburg)
In German Patent 197 01 751 the preparation of an Al2O3 microhollow fiber is described which was obtained by spinning of an aluminum oxide precursor [Al2(OH)5Cl], but the disadvantages of the method that were addressed are clear. For one, not all of the necessary starting materials are available. For another, the proportion of organic binder phase in the fibers is so large that sintering of the hollow fibers to produce defect-free ceramic fibers is not possible. In German Patent 197 01 751, individual fibers are heated to 1600° C., held there for one hour and slowly cooled again. Assertions for the defect-free nature of the fibers are likewise lacking like pictures of the calcined fiber. That merely a green fiber was recorded visually is indicative of the technical process difficulties of this method. From our own experiences it can be reported that the preparation of defect-free ceramic components is thus not possible. A variant of the spinning method is described by the same author in German Patent 19 730 996, in which no solution, sol or gel, but a ceramic melt is spun. In this a ceramic starting powder (Al2O3+aluminosilicate) at 2300° C. was fed through a workpiece nozzle and spun. The nozzle has to be made from a material that can withstand extremely high temperatures (tantalum or tungsten) which departs from the framework of standard, ready available nozzles. This variant of the spinning process produces, in the event that it is manageable, what surely does not represent an economic alternative for production of hollow fibers.
In order to be able to realize dense ceramic hollow fibers having small external and internal diameters and to realize with each economically desirable material, there are two possibilities, either one uses templates, which are removed in a second step and therewith bring about the transition fiber to hollow fibers or however one uses very small ceramic particles, which can then be manufactured into hollow fibers by the usual ceramic shaping methods like electrophoresis, extrusion or tape casting. The smaller the particles employed, the smaller are the fibers produced.
If however one wishes to fabricate ceramic hollow fibers for example for filtration, especially if one wants to advance in the areas of ultrafiltration or nanofiltration, then one must, either prepare the entire hollow fiber from nanoparticles (only in this way can one get into a fine pore sinter step) or however one must make a hollow fiber with layer structure. The latter implies a large pore carrier with a thin layer of nanoparticles to overlay. Commercially available high quality nanoparticles are either amorphous (SiO2) or like böhmite (AlO(OH)), precursors of real nanoparticles (aluminum oxide). During calcining of a multilayer system consisting of a porous carrier and a layer, of for example böhmite, the nanoparticle layer will always come off at the interface with the large pore carriers, since the temperature treatment initiates post-crystallization of the nanoparticles, which triggers a strong shrinkage and severe stresses, which for its part destroys the part. Since there are no commercial high quality (of redispersible primary particle sizes) crystalline nanoparticles, this problem has not been solved.
In summary this means that in addition to the preparation of very small geometries and also the preparation of small ones for filter elements requires the processing of very fine particles. The utilization of sub-micron particles is certainly suitable for this and especially suited I the utilization of nanoscale ceramic particles preferably having primary particle sizes smaller than 100 nm and especially preferred smaller than 20 nm. For realization of ultrafiltration and nanofiltration membranes or for the preparation of flexible, ceramic fibers the use of nanoparticles is indispensable.
The manufacture of ceramic hollow fibers by extrusion of fine particles is described in WO 99/22852. Here a submicron α-Al2O3 powder is employed in order to produce hollow fibers for the filtration area. According to the authors' data the powder is equipped with common commercial binding agents, extruded and calcined at 1300° C., wherein porosity of 35% is produced. The hollow fiber realized in the example had, before the sintering, an external diameter of 3 mm and an internal diameter of 2 mm, after sintering the external diameter was 2.4 mm and the internal diameter shrank to 1.6 mm. The hollow fiber till possessed a porosity of 35% and served for realization of ceramic filters. The linear shrinkage for a powder having a mean specific surface area of 10 m2/g amounted to 20%. Analogous to this U.S. Pat. No. 5,707,584 by the same authors is to be considered, wherein the authors attempt in their Claims to patent an external diameter between 500 μm and 3 mm. From our own experience it can be said that with the method that is described, external diameters of 500 μm are not achievable, an external diameter of 1 mm appears to be the lower limit. Furthermore it appears certain that the Claims only encompass porous hollow fibers, which can be used exclusively for filtration.
In a 1998 published paper [Werkstoffwoche 1998, 12-15 Oct. 1998, Munchen] Gut et al (EMPA) describe their progress in the production of hollow fibers by means of extrusion. So, the manufacture of ceramic hollow fibers using different materials is described in the sub-micron range, wherein the external diameter of the extruded hollow fibers is 150 μm and the internal diameter is 90 μm. The use of very fine nozzles leads to clogging of the tip by agglomerates or oversize grains. An additional problem was de-mixing, which occurred after individual data, due to poor chemical balancing of the powder/binder interaction. Also the preparation of densely sintered hollow fibers is only described in a single case, apart from that this does not succeed in what was adequate merely for use in filter systems.
In summary it can be recorded that the preparation of ceramic hollow fibers enjoys high industrial interest, wherein miniaturization is an advantage for many fields of application, or as the case may be, is decisive for many applications. The preparation of ceramic hollow fibers presupposes the availability of suitable powders for the application, likewise a suitable manufacturing method and sintering to defect-free component. Conventional spinning methods starting from solutions, brines and gels as shaping methods do not enter into consideration since the precursors used here with the high binder content, cannot be converted into hollow ceramic fibers, but at best as the Fraunhofer Gesellschaft has shown, into glass-type hollow fibers. Since the template method has not been mastered economically in these large quantities, only the classical ceramic shaping methods like electrophoresis (experimental stage), tape casting (thin foils must be rolled and bonded) or the extrusion (direct supply in the shape of tubes). The latter are also already used for producing hollow fibers or small ceramic tubes, wherein here the limit in respect to miniaturization has been reached, which depends on the minimum available stable particle sizes of materials employed. The smallest, patented hollow fibers have external diameters above 500 μm, the smallest hollow fibers known from the literature have an external diameter of 150 μm and a lumen (inner diameter) of 90 μm. All known hollow fibers are built from microscale particles and are usually porous, since technical process difficulties make it impossible to achieve close to theoretical density by sintering. The processing of nanoparticles to ceramic hollow fibers has not yet been described and can be viewed as new.
In order to manufacture defect-free hollow fibers for filtration in the area of ultrafiltration or nanofiltration, pore sizes <100 nm, preferably <50 nm and especially preferred <10 nm are necessary, which may be achieved only through the use of nanoparticles. Likewise, only through the processing of nanoparticles can miniaturized hollow fibers with outer diameters of <500 μm, preferably <200 μm and especially preferred <100 μm, be realized. The only ceramic forming method which directly provides the tubular shape is extrusion. For the extrusion ceramic masses must be developed from nanoparticles, whose solid state content >30 vol %, better <35 vol %, since otherwise the hollow fiber are exposed to high stresses during calcination and can be damaged. So that the process is economical to carry out, the manufacture of ceramic masses should moreover be carried out using conventional ceramic processing aggregates under conventional industrial conditions. These requirements that ceramic masses are processed to hollow fibers, based on nanoparticles with degrees of filling and on conventional processing aggregates goes far beyond the state of the art and up to now have not been realized.
The difficulty lies in the processing of nanoparticles. For particles with a particle size of about 10 nm, the specific powder surface area is increased to 250 m2/g. In connection with this the proportion of organic binder must be drastically increased, since the large surface area that is present binds the organic process-aid agent which then is no longer available for adjustment of the rheology. This again leads to very small solids contents in which for example in the extruding pastes whereby the linear shrinkage likewise like the stresses in the component are so large during sintering that all ceramic components such as for example the hollow fibers get destroyed. In the literature therefore only a small amount of information at all is found, for the processing of nanoparticles to ceramic components, since the difficulty always exists, to have sufficiently high solid material content for the sintering. While the processing of the powder particles by means of slip casting, electrophoresis and tape casting is often possible with low solid state contents (suspensions are processed), for ceramic shaping methods like screen printing, injection molding and extrusion ceramic pastes are prepared having suitable rheology and with high solids contents of >30 vol %, however preferably >35 vol %.
The smallest particle sizes, known from the literature for example are still processed by injection molding, have a particle size of 70 nm [Song and Evans, J. Rheologie 40, 1996, 131 ff]. Below 70 nm the primary particle sizes increase drastically and can amount to 250 m2/g for 10 nm particles. The increased interactions thereby between the particles and the organic processing aid and the higher viscosity related thereto reduce the solid state content so drastically, that injection molding is no longer possible. The extrusion of nanoparticles may be seen as analogous to this, which is likewise not known. In the case of silk screening, the preparation of suitable pastes based on nanoparticles is even more difficult, since in the extrusion and injection molding for the dispersion of the nanoparticles, the input of extremely high shear forces to the mold aggregate is possible in principle. For the paste preparation for the silk screening this is not possible, since the organic process aid material used there is usually not stable to shear stress. So, Kawahara et al [Key Engineered Materials, Vol. 159-160, 1999, pp 175-180] describe the situation in the silk screening of nanoparticles as follows. The larger the specific surface area of the nanoparticles is, then the more organic additives are needed for adjustment of the right paste rheology, since otherwise the paste viscosity is so high so that it is no longer processable. Since then again the amount of organic processing aid is too high, this leads to tears and defects t during burning off of the organic. The state of the art in the area of ceramic screen printing with nanoparticles is for example Carolla et al [Adv. Mater. 1999, 11 No. 11] The batches of nanoscale titanium dioxide to prepare with maximal contents of 5.4 Vol % (18.6 wt %), or Volkel et al [Symp. 7 Werkstoffwoche 1996 (1997) 601 ff] whose batches possess a maximal filler material content of 7.7 wt %. The best result in the literature for a ceramic mass from nanoscale particles which were processed by means of silk screening was for a solid state content of 17 vol % (55 wt %). With all these batches it is impossible to produce ceramic structures by means of silk screening.
The object of the present invention consisted of providing a ceramic batch based on nanoscale particles and a method for its manufacture, in which the solid state content of the nanoparticles and thus the powder content of the batch is so high that it can be processed by means of ceramic extrusion to produce hollow fibers. The hollow fibers thus manufactured should after the extrusion possess an external diameter of <500 μm, but preferably <200 μm and especially preferred <100 μm and allow them to be transformed in a downstream process into ceramic hollow fibers. The hollow fibers produced in this manner should each, according to the application area, be porous or be sintered to close to theoretical density.